1. Field of the Invention
The present invention relates to a display apparatus for displaying images formed on an image bearing member.
2. Description of the Prior Art
Cathode ray tube (Braun tube), liquid crystal display devices, etc. have conventionally been employed as an image display apparatus, for example, in the terminals for displaying as a visible image information which is supplied in the form of electrical signals from a computer or an image reader or which is accumulated in a magnetic tape or the like, or in the image monitors for use in office automation equipment such as a copier or other image-related equipment.
Among these, the cathode ray tube display apparatus is most commonly employed and is highly reliable, but is unable to display small characters or fine images in the actual size as in the newspapers or magazines due to a limitation in the resolution power. In practice an enlarged display is employed for ensuring legibility, but the number of characters displayed in the display frame, or the amount of information per frame, decreases inversely proportional to the rate of magnification. Also a prolonged continuous display of a same image results in a deterioration in the displaying ability. In addition eye fatigue is caused by the flickering of the display in prolonged use. On the other hand, the liquid crystal display, which has been recently developed, is still associated with an insufficient resolution power as in the cathode ray tube, and is difficult to be manufactured and expensive in a large size.
As a third image display apparatus not associated with the above-mentioned drawbacks, the present applicant proposed, in the Japanese Patents laid-open No. 98746-98749/1983, an image display apparatus employing an electrophotographic process. Said image display apparatus, in which the image information is displayed as a toner image, is capable of providing an easily legible image with a sufficient resolution power, is relatively inexpensive even in a large size, and is highly reliable. In the image display apparatus employing electrophotographic process, an electrophotographic photosensitive member formed for example as an endless belt is scanned with a light beam modulated by image signals to form an electrostatic latent image on said photosensitive member, then said latent image is developed with toner to obtain a toner image and the photosensitive member bearing said toner image is moved to a display station for visual observation. After the completion of display, the photosensitive member is moved to a cleaning station for removing the toner image so that the photosensitive member can be used repeatedly.
However, in such image display apparatus employing electrophotographic process, the photosensitive member constituting the display plane has to move between an image forming station and a display station in contrast to the image display system in which the display plane is fixed in a position as in the cathode ray tube, and the formed image is erased when the photosensitive member is moved. Consequently it is not possible, when the currently displayed image is related with the previously displayed image, to observe these two image at the same time.
For example, in case a patent specification and an attached drawing are given as separate images, it is necessary to display the specification first and then to move the photosensitive member for displaying the drawing to make reference between the two, and the lack of simultaneous display of the two images has been extremely inconvenient for the user.
Furthermore, several seconds are required before a new image is displayed after the observation of a preceding image, so that the observation of plural images in succession necessitates a very long time and is therefore inefficient for the user.